Mendeley offers word processing plug-ins for Microsoft Word and LibreOffice.To install the plug-ins, go to the Tools menu on the Mendeley Desktop version. In the drop down menu, click on the appropriate line to "Install MS Word/LibreOffice Plugin"Once you download and install Mendeley Desktop to a computer, you can install the Microsoft Word Plugin. Open the Desktop, then click on Tools, then Install MS Word Plugin. You may have to close and reopen Word for the install to be completed.
Depending on your operating system, the MS Word Plugin will appear differently. If you are using Word for Mac, you will have a floating bar or an Add-in, depending on your version of Word (Pictured: Word 2016 for Mac):
If you are using Word on Windows, the Mendeley Plugin will be integrated in the Word Ribbon. Unfortunately it closely resembles the Built-in "Insert Citation" tool in the Ribbon, so be careful to choose the "Insert Citation" with the Mendeley icon on it (Pictured: Microsoft Word 2013):